Polyesters have been produced commercially on a large scale for processing into shaped articles such as fibers, primarily from poly(ethylene terephthalate). Synthetic polyester yarns have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251, and then, in U.S. Pat. No. 2,465,319, Whinfield and Dickson suggested poly(ethylene terephthalate) which is the synthetic polymer most widely manufactured and used hitherto for textile fibers and which is often referred to as homopolymer PET. Homopolymer PET has generally been preferred over copolymers because of its lower cost, and also because its properties have been entirely adequate, or even preferred, for most end-uses. It is known, however, that homopolymer PET requires special dyeing conditions (high temperature requiring super-atmospheric pressure) not required for nylon fibers, for example, so copolyesters have been suggested and used commercially for some purposes, e.g., cationic-dyeable copolyesters such as have been disclosed by Griffing and Remington in U.S. Pat. No. 3,018,272 and by Hansen et al. in U.S. Pat. Nos. 5,171,309 (DP-6335) and 5,250,245 (DP-6335-B).
Polyester fibers are either (1) continuous filaments or (2) fibers that are discontinuous, which latter are often referred to as staple fiber or cut fibers, and are made by first being formed by extrusion into continuous polyester filaments, which are processed in the form of a tow of continuous polyester filaments before being converted into staple. An important stage in the processing of continuous polyester filaments has been "drawing" to increase the orientation of the long chain polyester molecules, and thereby improve the properties of the filaments.
Mostly, the objective of synthetic fiber producers has been to replicate advantageous properties of natural fibers, the most common of which have been cotton and wool fibers. Most polyester cut fiber has been homopolyester PET of round cross-section and has been blended with cotton. Homopolymer PET is hydrophobic, whereas cotton absorbs moisture, and cotton fabrics have heretofore been preferred over fabrics of synthetic polymers by many people because they have believed that many cotton fabrics have been more comfortable to wear than most fabrics of most synthetic polymer fibers, which have mostly been of round cross-section as previously stated herein. Filaments of round cross-section are the easiest and most economical synthetic filaments to spin and dye, which is why practically all synthetic filaments have been of round cross-section, except for specialty filaments which are more expensive to make and more expensive to dye because of their increased surface area.
For several years, homopolymer PET fibers of generally scalloped-oval cross-section with grooves that run along the length of the fibers have been available commercially from E. I. du Pont de Nemours and Company and have given significant advantages over both cotton and over homopolymer PET fibers of round cross-section because of the increased comfort properties provided by this polyester fiber having (non-round) cross-section. Their longitudinal grooves have provided increased moisture-wicking advantages over fibers of round cross-section, and the fact that moisture is wicked along the fibers instead of being absorbed has been an advantage in contrast to cotton. Further advantages would, however, be desirable and obtainable according to the present invention.
Recently, U.S. Pat. Nos. 5,591,523 (DP-6255) and 5,626,961 (DP-6365-A) and copending application Ser. No. 08/662,804 (DP-6400) filed Jun. 12, 1996, and now allowed, corresponding respectively to WO 97/02372, WO 97/02373 and WO 97/02374, the disclosures of which are hereby incorporated herein by reference, have disclosed inventions relating to polyester tows that are suitable for conversion to slivers on a worsted or woollen system and downstream processing on such systems, eventually into fabrics and garments. The present invention has been made in the course of that work, and is described with particular reference to its value in drawing polyester filaments in tows. The tows that were disclosed in U.S. Pat. No. 5,591,523 consisted essentially of polyester filaments of scalloped-oval cross-section with grooves that run along the length of the filaments and were mixtures of filaments of higher denier per filament and of lower denier per filament with specified ranges and were suitable for processing on a worsted or woollen system. In addition to tows that are suitable for processing on a worsted or woollen system, it would be desirable to provide polyester fibers for processing on a cotton system, such as are processed entirely differently.
Cotton system processing is performed on cut staple polyester fiber and, of course, on cotton, which is a natural fiber of similar length to the cut fiber of polyester staple. Staple fiber is usually sold and packaged in compacted bales, as opposed to processing on a woollen or worsted system. The bales are opened and the fibers are conveyed on a pneumatic or mechanical system to a card. The card breaks up tufts of fibers, aligns them into a web of parallelized fibers which are formed into a continuous sliver as it leaves the card. The sliver may then be blended with other fibers such as cotton on a draw frame, and is passed through one or more additional draw frames to improve the blend and along end uniformity. The sliver is then spun into yarns on a spinning system, such as an open-end spinning frame, air jet spinning frame or ring spinning frame. In some cases, the sliver from the draw frame is converted into roving to further reduce the sliver weight, before being spun into yarn on a ring spinning frame to make yarn of appropriate size (count) and level of twist prior to fabric formation.
As, for example, has been disclosed in U.S. Pat. No. 5,591,523 (DP-6255), filaments (of generally scalloped-oval cross-section and) of different denier per filament (dpf) were desired, and surprise was expressed in Example 1 of that patent that it was possible to spin undrawn homopoly(ethylene terephthalate) (modified with tetraethyl orthosilicate) filaments that had been spun of significantly different denier on the same spinning machine without adjusting the natural draw ratio and then subsequently to draw an intimate mixture of these spun filaments simultaneously in the same tow at the same draw ratio to provide filaments with excellent properties that were different because of their differing dpfs (col 6, lines 15-29). The present invention expands on this surprising finding and extends it to the drawing simultaneously of bundles of mixed filaments that were not specified in that patent.